In the manufacture of optical fiber, a glass preform rod which generally is manufactured in a separate process is suspended vertically and moved into a furnace at a controlled rate. The preform softens in the furnace and optical fiber is drawn freely from the molten end of the preform rod by a capstan located at the base of a draw tower.
Because the surface of the optical fiber is very susceptible to damage caused by abrasion, it becomes necessary to coat the optical fiber, after it is drawn, before it comes into contact with any surface. Inasmuch as the application of a coating material must not damage the glass surface, the coating material is applied in a liquid state. Once applied, the coating material must become solidified rapidly before the optical fiber reaches the capstan. This may be accomplished by photocuring, for example.
Those optical fiber performance properties which are affected most by the coating material are strength and transmission loss. Coating defects which may expose the optical fiber to subsequent damage arise primarily from improper application of the coating material. Defects such as large bubbles or voids, non-concentric coatings with unacceptably thin regions, or intermittent coatings must be prevented. The problem of bubbles in the coating material has been overcome. See, for example, U.S. Pat. No. 4,851,165 which issued on Jul. 25, 1989 in the names of J. A. Rennell, Jr. and C. R. Taylor. Intermittent coating is overcome by insuring that the fiber is suitably cool at its point of entry into the coating applicator to avoid coating flow instabilities. Coating concentricity can be monitored and adjustments made to maintain an acceptable value.
Optical fibers are susceptible to a transmission loss mechanism known as microbending. Because the fibers are thin and flexible, they are readily bent when subjected to mechanical stresses, such as those encountered during placement in a cable or when the cabled fiber is exposed to varying temperature environments or mechanical handling. If the stresses placed on the fiber result in a random bending distortion of the fiber axis with periodic components in the millimeter range, light rays, or modes, propagating in the fiber may escape from the core. These losses, termed microbending losses, may be very large, often many times the intrinsic loss of the fiber itself. The optical fiber must be isolated from stresses which cause microbending. The properties of the optical fiber coating material play a major role in providing this isolation, with coating geometry, modulus and thermal expansion coefficient being the most important factors.
Typically two layers of coating materials are applied to the drawn optical fiber. Furthermore, two different kinds of coating materials are used commonly. An inner layer which is referred to as a primary coating material is applied to be contiguous to the optical glass fiber. An outer layer which is referred to as a secondary coating material is applied to cover the primary coating material. Usually, the secondary coating material has a relatively high modulus, e.g. 10.sup.9 Pa, whereas the primary coating material as a relatively low modulus such as, for example, 10.sup.6 Pa. In one arrangement, the primary and the secondary coating materials are applied simultaneously. Such an arrangement is disclosed in U.S. Pat. No. 4,474,830 which issued on Oct. 2, 1984 in the name of C. R. Taylor.
Subsequently, both the inner and the outer layers of coating materials are cured beginning from the outside progressing inwardly. Also typically, the primary and the secondary coating materials comprise ultraviolet light curable materials each being characterized by a photoactive region. A photoactive region is that region of the light spectrum which upon the absorption of curing light causes the coating material to change from a liquid material to a solid material. Both the materials which have been used for the primary and for the secondary materials have comparable photoactive regions. Because the photoactive regions are comparable, the curing light for the primary coating material will be attenuated by the secondary coating material. As a result of the attenuation, less light reaches the primary coating material.
Of course, notwithstanding the attenuation of the curing light by the secondary coating material, it is important that the primary coating material be fully cured. This problem has been overcome in the prior art by reducing the line speed to allow longer exposure time of the primary coating material to the ultraviolet curing light energy inasmuch as the ultraviolet curing light energy is inversely proportional to line speed.
Although the foregoing solution is a workable one, it has its shortcomings. Most importantly, any reduction in line speed is not desirable and runs counter to current efforts to increase draw lengths and to increase substantially draw speeds of the optical fiber.
What is needed and seemingly what is not disclosed in the prior art is a coated optical fiber which overcomes the foregoing problem of attenuation by the secondary coating material of the light energy used to cure the primary coating material. Any solution should be one which does not affect adversely the line speed. Further, methods which must be implemented to make such a sought after coated optical fiber must be capable of being integrated with present manufacturing arrangements for drawing optical fiber from a preform.